Transparent displays are long sought-after by scientists and engineers. Two-dimensional (2D) transparent displays can create images that appear floating in the air, in contrast to traditional displays where images appear on a visible screen. Aside from creating special visual impressions, such displays can have a wide variety of applications. A glass window can be turned into the screen of a home theater. Eyeglasses can become a mini computer screen. The windshield of a vehicle can show information, such as maps, without blocking the driver's view. The display window of a store can show not only products but also their information.
The most common transparent displays, such as those used in some airplanes, cars, and Google Glass, are Heads-Up Displays (HUDs). An HUD creates images by projecting light onto glass, which reflects the light beam into the viewer's eyes via specular reflection. A drawback of these displays is that specular reflection typically occurs at only one angle. As a result, a viewer can only see the images from a limited range of angles, i.e., the viewing angle is small.
Another type of transparent display is the holographic screen (also referred to as a diffusion screen), which includes a diffractive element (also referred to as a holographic element) to scatter light. The viewing angle of a holographic screen is usually larger than that in HUDs but is still not satisfactory for a broad audience. In addition, the transmittance of holographic screens is typically very low and the laser writing process to fabricate the diffractive element is complex and costly.